Monaco-editor: Provide straight-forward zoneWidget API

[robclive]

I'm trying to create a view zone similar to the one which shows up while displaying errors:

I used changeViewZones to insert a viewzone but it creates a zone which spans from the end of line gutter to the end of the editor. I want it to start from the start of the line gutter.

Is there any way I can reuse what is being used in the error viewzone. If not, how can I mimic it.

[alexdima]

The go to error widget does two things:
1) inserts a view zone to reserve a vertical gap in the text
2) inserts an overlay widget that is kept position-wise in sync with the view zone

The trick is that when inserting a view zone, it is possible to provide two callbacks: onDomNodeTop and onComputedHeight. The callbacks provide the top and the height of the view zone. These values can then be used to position correctly an overlay widget.

Internally, we have something called a zoneWidget, and I think there are many use-cases for this. We should provide straight-forward API for the zoneWidget.

[hadrienk]

Until the zoneWidget API is available this is an example of how you could implement the trick explained above.

     // Create a zone over the margin. Uses the trick explained
    // at https://github.com/Microsoft/monaco-editor/issues/373

    // overlay that will be placed over the zone.
    let overlayDom = document.createElement('div');
    overlayDom.id = 'overlayId';
    overlayDom.style.width = '100%';
    overlayDom.style.background = '#ffb275';

    // https://microsoft.github.io/monaco-editor/api/interfaces/monaco.editor.ioverlaywidget.html
    let overlayWidget = {
      getId: () => 'overlay.zone.widget',
      getDomNode: () => overlayDom,
      getPosition: () => null
    };
    editor.addOverlayWidget(overlayWidget);

    // Used only to compute the position.
    let zoneNode = document.createElement('div');
    zoneNode.style.background = '#8effc9';
    zoneNode.id = "zoneId";

    // Can be used to fill the margin
    let marginDomNode = document.createElement('div');
    marginDomNode.style.background = '#ff696e';
    marginDomNode.id = "zoneMarginId";

    editor.changeViewZones(function(changeAccessor) {
      changeAccessor.addZone({
        afterLineNumber: 3,
        heightInLines: 3,
        domNode: zoneNode,
        marginDomNode: marginDomNode,
        onDomNodeTop: top => {
          overlayDom.style.top = top + "px";
        },
        onComputedHeight: height => {
          overlayDom.style.height = height + "px";
        }

      });
    });

[samghelms]

If anyone wants another example of how to use changeViewZones, check this out: https://microsoft.github.io/monaco-editor/playground.html#interacting-with-the-editor-listening-to-mouse-events